pdfork(2): add PD_PTRACE_CAP flag

If the flag is not specified, the process descriptor returned by either
pdfork(2) or pdopenpid(2) has the CAP_PTRACE capability disabled.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D58586